Today's market for kiln dried lumber demands significantly greater attention to the lumber production process. Government and industry regulations and customer expectations of wood quality have increased every year. Additionally, competitive pricing of final lumber grade has become more severe. Therefore, mills must maintain a high level of scrutiny of each production stage to reduce or eliminate product errors and waste. Process mistakes that do occur result in wood products that have compromised strength properties, may be susceptible to mold, or can lose significant value due to shrinkage or any number of visual defects. In addition, wood processing errors can cause lost productivity as well as higher energy costs for a mill.
There are a number of vendors that provide in-kiln moisture measurement systems. In fact, prior art systems have been commercially available for two decades. Such prior art systems (e.g., by the manufacturers Wagner, Wellons, and Accudry, SCS Forest Products) have been described in significant detail in various public disclosures, including the following U.S. Pat. No. 4,389,578 by Wagner; U.S. Pat. No. 4,580,233 by Parker, et al.; U.S. Pat. No. 6,703,847 by Venter et al.; and U.S. Pat. No. 6,989,678 by Venter et al. each of which is fully incorporated herein by reference. Additionally, the following U.S. patent numbers are fully incorporated herein by reference: U.S. Pat. No. 3,807,055 by Kraxberger; U.S. Pat. No. 4,107,599 by Preikschat; U.S. Pat. No. 6,124,584 by Blaker et al.; U.S. Pat. No. 6,281,801 by Cherry et al.; U.S. Pat. No. 6,784,671 by Steele, et al.; U.S. Pat. No. 6,784,672 by Steele, et al.; U.S. Pat. No. 7,068,050 by Steele; and U.S. Pat. No. 7,068,051 by Anderson. U.S. Patent Application Publication No. US 2004/0187341 by Studd, et al. is also fully incorporated herein by reference.
All of these systems work using decades-old theory by converting capacitance response to moisture content. This is accomplished by placing two or more metal plates in a lumber stack, which are vertically separated by some calculated distance. The system then creates a capacitor by applying an electrical signal to the plates. The main dipolar constituent between the plates is water within the lumber stack. Therefore, the capacitance response diminishes during a drying cycle as water is removed. The drop in capacitance is correlated to the loss of moisture.
To-date, the prior art systems that perform the function of measuring the change in capacitance over time, have relied upon placing one or more fixed wall-mounted metering devices inside, outside or near the kiln for drying the lumber therein. The metering devices are responsible for determining the electrical response of the circuit formed by the metal (steel) plates and the lumber stack between the two plates. Cables connect the meters to the plates inserted in the lumber stack. The metering devices, in turn, are connected, via wire or cables, to a central programmable logic controller (PLC) or other computing device (e.g., a personal computer, PC herein). The calculation of moisture content is performed either at the meters themselves or at a central PLC or PC. These wired systems have been installed throughout lumber drying kilns in North America.
These prior-art lumber drying systems have a number of limitations. Since all of these systems are connected via cables to a main PC or controller, the number of meters is limited by financial constraints. In addition to the per-unit cost for each meter, conduit runs must be installed to protect cables that transmit electrical measurements to the PCL or PC. Depending on the configuration of the kiln, these conduit runs can be hundreds of feet long. Running high temperature cable inside aluminum conduit between each meter and the PLC or PC is a considerable upfront expense as well as an on-going maintenance expense.
Another limitation of such prior art lumber drying monitoring systems is that once a measurement point (e.g., meter) is fixed to the kiln wall and conduit is run to that point, the location cannot be easily changed without incurring significant cost. Therefore, the operator does not have flexibility to target desired locations in the kiln drying lumber. Often times, lumber mills will produce new lumber products that require greater in-kiln observation in the first number of manufacturing cycles. Installing additional moisture sensors in a timely manner is not an option. Therefore, mills will often incur costly production loss and greater energy usage in the early manufacturing runs until the manufacturing process has been standardized.
Finally, the entire lumber production industry is undergoing a significant shift as the industry moves from batch processing of lumber to continuous processing. Instead of packages of lumber being placed inside a kiln for a set period of time, operators are continuously moving the stacks of lumber on rail tracks through various heating chambers. Having measurement sensors within such lumber stacks wherein the sensors are tethered to meters of fixed location is not an option in this case. In particular, the lumber stacks may travel approximately 100 feet (or greater, e.g., 200 feet) through the various drying chambers. At any one time, there may be 10 or more distinct lumber stacks moving through the drying process. Moving cables attached to each lumber stack would create too many safety and logistical issues for the mills to consider this a viable option.
Regarding wireless technology, unfortunately, commercially available in-situ wireless moisture measurement sensors are not an option for a number of reasons. First, there are no viable wireless systems that can survive softwood kiln temperatures. In most cases, kiln temperatures reach 260° Fahrenheit (F.) or higher, which is far higher than conventional moisture sensor technology allows. Second, the kiln environment has very high humidity with hot ash and sap sticking to any available surface. Because of these conditions, very limited electronics have heretofore been provided inside the kiln. In particular, most commercially available electronics have a maximum temperature of 125° F. and are not effective for marine environments (e.g., where there is consistently high moisture content of, e.g., 90% or more). Further, in-kiln temperatures can range from −40 to +260° F., and this wide temperature range is especially problematic in that kiln temperatures can ramp from the low end of this range to the upper end of this range in a matter of hours (e.g., three to four hours or less). Furthermore, sensors (in the lumber stacks) and the meters placed in the kiln must operate continuously for, e.g., up to three weeks.
Moreover, if the moisture sensors within the lumber stacks are to be untethered (e.g., wireless) in their communications, then they must be powered by batteries. However, in general, battery life for electronic devices is severely degraded by the elevated kiln temperatures as recited above. In fact, studies show that batteries operating at temperatures above 113° F. will lose 50% of their useful operating life performing a task whereas at lower temperatures (e.g., 100° F.), there would only be a 20% to 30% reduction in such useful operating life. This is particularly important for the drying and processing of softwood lumber since such lumber may need to be monitored for lengthy time periods, e.g., approximately six months or longer in kiln environments with extremely high prolonged temperatures and/or extremely high prolonged moisture content. Accordingly, perhaps the most challenging for wireless lumber monitoring sensors is the battery life. This is probably the primary reason that no other manufacturer has developed a wireless sensor based capacitance system for the monitoring the moisture content of softwood within wood drying kilns since softwood lumber in-kiln drying requires sensor batteries to be operationally useful at prolonged temperatures of 260° F. (or higher)—significantly higher than the top range for standard batteries to effectively operate, e.g., a typical wireless sensor. In particular, such high temperatures may be required for up to twenty-one days. It is, however, worth mentioning that there are specialty batteries that operate at higher temperatures, but battery life would still be a significant issue.
Because there are no wireless options for the softwood market, meter suppliers have looked at creating physical connections using sleds instead of cables between the meters and the lumber stack. As the lumber stack moves past a fixed sled, the sensors within the lumber stack come into electrical contact (e.g., via a protrusion from each sensor contacting sled) the meter can make a valid measurement. There are a number of disadvantages with such sled systems, including, but not limited to installment cost, maintenance costs, accuracy and limited lumber moisture sampling ability due to the sleds being attached to the kiln wall. Accordingly, the adoption of these sleds has been very slow.
Due to the drawbacks (e.g., as recited above) with the prior art lumber drying monitoring systems for the lumber industry, the technology in the present disclosure has been developed for addressing such drawbacks, and in particular, providing an apparatus (e.g., one or more computational devices/equipment) and computer methods for monitoring lumber characteristics (e.g., moisture content), wherein wireless lumber measurements are taken by sensors embedded within lumber stacks and such sensors can remain operationally effective for extended periods of time without maintenance such as battery replacement.
Accordingly, it would be advantageous to have a lumber monitoring system and method that mitigates or cures the above-identified drawbacks of the current lumber drying systems and methods.